1. Field of the Invention
The present invention relates to a lens apparatus for use in a still camera, video camera, television camera for broadcasting or the like that is provided with a separating optical unit disposed in an optical path and is adapted to perform automatic focusing (AF: Auto Focus) utilizing a separated light beam.
2. Description of the Related Art
Heretofore, various techniques of automatic focusing in shooting apparatuses such as still cameras and video cameras have been developed. In particular, Japanese Patent Application Laid-Open No. 2004-85673 and Japanese Patent Application Laid-Open No. 2005-181469 disclose lenses and image pickup apparatuses in which a separating optical unit is provided in an optical path and an AF detection unit is provided in the separated optical path.
These systems suffer from a problem of decrease in the light amount in the shooting optical system involved by providing a focus detection system separately from a shooting optical system. To solve this problem, Japanese Patent Application Laid-Open No. 2003-270520 and Japanese Patent Application Laid-Open No. 2004-118141 disclose an optical system in which the reflectance of a separating optical unit which introduces light into the focus detection system is changed by turning a turret to achieve a required amount of light in the image pickup system, and an optical system having a focus detection system that uses light in the infrared range.
In conventional focus determination methods that use a separating mirror, it is necessary to introduce a certain amount of light into a focus detection system, which leads to a decrease in the transmittance in the image pickup system, which in turn leads to an increase in the T-number. In particular, when the brightness of the object is low, there is a problem that an amount of light necessary in shooting cannot be obtained even when the aperture stop is fully opened.
Furthermore, when the turret is turned to change the reflectance of the separating mirror, there is a problem that the reflectance cannot be changed during shooting since the image is interrupted for a while.
In order to eliminate the above described problems, the present invention provides a lens apparatus that can operate with high precision at high speed while separating incident light beams in an imaging optical system to perform focus detection without a decrease in the amount of light in shooting.